1. Field of the Invention
This invention relates generally to battery chargers and, more particularly, to battery chargers that control the rate of charging to determine when batteries have reached full charge.
2. Description of the Related Art
Rechargeable batteries, such as nickel cadmium batteries, are popular because they provide a ready source of electrical energy and avoid the waste associated with the disposal of batteries that cannot be recharged. When rechargeable batteries are discharged, they are connected to a charging system that restores them to a charged condition. In a conventional fast-charge system, for example, a current that is equal to the battery charging capacity C in ampere-hours is delivered to the battery for one hour. This charging rate leaves the battery with approximately a full charge.
Conventional charging systems can damage batteries by overcharging them, or can waste time and energy by undercharging them. Undercharging is a condition in which the electrical energy delivered to a rechargeable battery is not sufficient for the battery to deliver its rated power. As a result of undercharging, a battery probably will not be capable of operating equipment with which it is used and will have to be recharged more frequently. Repeated recharging takes the battery out of service and is a great inconvenience. Overcharging is a condition in which electrical energy continues to be delivered to the battery after it has reached full charge. As a result of overcharging, the battery cycle life is reduced, and the battery likely will need to be replaced much earlier than anticipated. A number of charging systems are designed to charge batteries without damage, and to provide fast and safe charging up to the full rated power of the batteries, by monitoring indicators of full charging.
Battery temperature and voltage can be used to indicate full charge. Both the temperature and internal resistance of a discharged battery increase as the battery is charged. Some charging systems monitor the temperature of a battery and attempt to prevent overcharging by stopping the charging process when a maximum allowed temperature is reached. This temperature, however, might not correspond to the maximum battery temperature reached during charging. For example, the maximum temperature reached by a charging battery can vary depending on ambient temperature and on the condition and age of the battery. Selecting battery temperature as the charge indicator to be monitored is made more difficult by the need to carefully select and locate temperature sensors to ensure accurate monitoring.
The monitored battery temperatures themselves can be misleading. Fluctuations in the temperature of the battery can occur, resulting in momentarily elevated temperatures in the absence of a full charge. The frequency and magnitude of the fluctuations can vary with the environmental conditions and the condition of the battery. Thus, monitoring the battery temperature for a maximum value can result in premature completion of charging and, therefore, undercharging rather than overcharging.
The battery voltage also increases with the charging of the battery, reaching a peak and then rapidly decreasing. Although monitoring battery voltage avoids the problems of ambient temperature, sensor placement, and influences of adjacent batteries, it still can fall prey to voltage fluctuations and can result in premature completion of charging. The voltage fluctuations occur in a manner similar to the temperature fluctuations and can result in momentarily high voltages that do not represent the peak value. The momentary voltage changes are often interpreted as peak charging values by typical charging systems that monitor voltage. Such charging systems respond to the momentary voltage peaks by quickly halting charging, often resulting in an undercharged condition. On the other hand, a system that does not respond to voltage changes in a sufficiently short time period can overcharge a battery.
Finally, many charging systems can only charge a single battery at a time, and cannot charge a plurality of batteries to their full charge. This is inconvenient, because many devices require multiple batteries for operation, and it is easier to simultaneously remove several discharged batteries from a device and replace them with charged batteries, while simultaneously placing the discharged batteries into a charging system.
From the discussion above, it should be apparent that there is a need for a battery charging system that charges rechargeable batteries to their full charge without overcharging them due to slow response and without undercharging them due to spurious indicators of full charge. The present invention satisfies this need.